This invention relates to access systems as used in wireline telephony and, more specifically, to a switch proxy for use in conjunction with an access system remote terminal to route telephone calls when communications between a remote terminal of an access system and its controlling switching system is lost.
Wireline telephone service providers use access systems to serve telephone subscribers that are not economically or practically served directly from the nearest local switching system. Examples of access systems of this type include subscriber loop carriers and digital loop carriers (or DLC's). An access system consists of a remote terminal that is connected to a local switching system (or central office or CO) by one or more digital trunk groups. The remote terminal can be located in a field cabinet, where the telephone subscriber's traditional copper pair is created in nearer proximity to the subscriber's premises. This capability improves the service that the subscriber receives and provides the telephone service provider with an economic alternative to long cable runs from the local switching system.
For engineering simplicity, remote terminals are typically designed to have very little independent operational capability. They rely on the host switching system for essentially all of the intelligence that would normally be associated with origination, routing and completion of a telephone call. The subscriber lines terminate at the remote terminal, which converts the voiceband signals into digital signals that are, in turn, multiplexed over digital channels on the trunk lines between the local switching system and the remote terminal. The trunk lines also carry separate digital channels for control information. The local switching system controls the remote terminal as if it were an extension of the switching system. For example, dial tone is sourced from the local switch and the dialed digits are collected by it.
In this manner, service providers are afforded more options in providing telephone service to subscribers. In particular, these access systems provide a much more economical approach to serve a small remote community of subscribers than the use of expensive local switching systems or proprietary remote switching modules. Typically DLC systems support between 100 and 2000 subscribers.
The simplicity of aggregating all of the call control functionality in the host switching system creates a problem in the art. That is, when the host switching system is unable to communicate with the remote terminal, either through failure of components of the digital trunks or of the switching system itself, subscribers served by the remote terminal no longer have any telephone service. Even though the connectivity with the greater network is lost and the remote terminal may be otherwise fully functional, the subscribers terminated on this remote system still cannot communicate with one another. The severity of this problem may be confounded by the fact that these subscribers are frequently served by this technology precisely because they are remote. These remote settings necessitate the use of local emergency responders and limit other communications options (e.g., cellular telephony). Therefore, the ability to continue to locally switch calls between subscribers served by remote systems that have lost communications with a host switching system is an important public safety consideration.
In urban deployments, the loss of connectivity to an access system may be only a minor inconvenience; nearby neighbors may be served from another system and cellular service will probably be available. Furthermore, urban densities allow for engineering the transport facilities with extra capacity that allows for “fail over” to protection facilities.
In rural settings, the remote terminal may provide the telephone services for an entire community. These rural communities could be very isolated from other communities and cellular service may be non-existent. Loss of communications between the host switch and the remote terminal in these circumstances could result in more serious consequences than in urban scenarios.
To mitigate the potential negative impact on public safety and communication among members of the community during such service outages, some rural public utility commissions and other agencies have promoted inclusion of Emergency Stand Alone (ESA) capabilities in telephone access systems. ESA capabilities typically include the ability for subscribers to dial 9-1-1 to reach public safety personnel and to complete telephone calls among the members of the local community isolated by the service outage.
The Public Safety Answering Point (PSAP) is normally the location where operators answer emergency calls and dispatch first responders. Usually, telephone services to a PSAP are provided directly from a central office and not from a DLC system. This is for a variety of reasons, including the fact that PSAPs generally require special trunks (e.g., CAMA trunks) to carry Automatic Number Identification (ANI) and Automatic Location Identification (ALI) information to emergency operators, and DLC systems typically do not support such special trunks. Therefore, isolated ESA systems (in emergency mode) will usually need to direct 9-1-1 calls to an alternative location, to a 9-1-1 designate, rather than to a PSAP. The 9-1-1 designate will, of course, need to be a subscriber served by the ESA system while in emergency mode, a sheriff's substation or a local fireman's home, for example, may be chosen for this responsibility.
Other suggestions in the prior art pertain to installation of a “miniature” switching system in the proximity of the remote terminal to serve as a local host. This approach is not only expensive but impractical on several counts:                i) it changes the basic architecture of the exchange network,        ii) it increases the number of switches to administer and maintain,        iii) it actually increases the probability of a service outage by putting another switching system into the chain, and        iv) these remote terminals are frequently installed in field cabinets where it may be impossible to install an additional complex system.Thus, there currently does not exist an economical or practical scheme for providing emergency stand alone service to subscribers served by the installed base of access systems. In addition, this invention discloses methods by which a 9-1-1 designate may be informed when incoming calls are emergency calls and be provided with information related to the caller similar to that available to a PSAP operator to insure proper handling of the call. In particular, the present invention addresses these issues by providing a ESA-capable system that enables 9-1-1 designates to be informed of the name, telephone number and address of emergency callers during these service outages.        